Group-III element nitride semiconductors have a high melting point and have a high nitrogen dissociation pressure at around the melting point. Consequently, bulk growth from the melt is difficult. Meanwhile, it is known that a Group-III element nitride semiconductor substrate can be produced by using vapor phase growth methods such as a hydride vapor phase epitaxy (HVPE) method and a metal-organic chemical vapor deposition (MOCVD) method.
For example, in the case of producing a gallium nitride semiconductor substrate, a base substrate such as, for example, sapphire is set in the growth chamber (reactor) of a vapor phase growth apparatus. Gases for Group-III element nitride semiconductor formation which include, for example, a gas containing a gallium compound and a gas containing a nitrogen compound are fed to the inside of the reactor to thereby grow a gallium nitride semiconductor to a thickness of several micrometers to several centimeters on the base substrate. Thereafter, a portion including the base substrate is removed by polishing or laser light irradiation. Thus, a desired Group-III element nitride semiconductor substrate can be obtained.
Of those vapor phase growth methods, the HVPE method has the feature of being capable of attaining a higher growth rate than other growth methods. The HVPE method hence is effective when it is necessary to grow a Group-III element nitride semiconductor in a large thickness, or is effective as a technique for obtaining a Group-III element nitride semiconductor substrate having a sufficient thickness.
Hitherto, one single-crystal gallium nitride substrate has been obtained by growing single-crystal gallium nitride having a thickness of about 500 μm to 1 mm on one base substrate made of a different kind of material, removing the base substrate, and subjecting the remaining portion to processing/polishing. This method, however, is exceedingly low in production efficiency and cannot be used to provide an inexpensive single-crystal gallium nitride substrate.
Attention is hence being focused on a technique in which a single crystal having a thickness of several millimeters to several centimeters is grown on a base substrate and this single-crystal bulk is sliced to thereby obtain a plurality of single-crystal gallium nitride substrates using the one base substrate.
In such a case, however, a problem arises when a film is grown to a thickness of several tens of micrometers to several hundred micrometers or more. When a film is grown to a thickness of several tens of micrometers to several hundred micrometers or more, cracks generate in the crystal during the growth. Consequently, a crystal having a large area is not obtained.
A technique has been proposed in which the shape of the base substrate is regulated in order to inhibit such crystal cracking. Specifically, patent document 1 describes a single-crystal substrate in which the main surface is a convex surface and has been mirror-polished, as a single-crystal substrate on which a film of a nitride semiconductor crystal of higher quality can be formed.
Patent document 2 describes that a base substrate which has a convex surface shape and has an amount of warpage of 20-100 μm is used in order to obtain a nitride semiconductor substrate reduced in in-plane off-angle unevenness.
Furthermore, in patent document 3, a Group-III element nitride semiconductor crystal substrate in which atoms of the Group-III element constitute a convex surface and which has a specific warpage ratio is described for the purpose of providing a Group-III element nitride semiconductor device having reduced unevenness of luminescence intensity.
Another technique for inhibiting such crystal cracking has been proposed, in which the amount of warpage of the crystallographic-plane shape (and crystal axis) of a base substrate is regulated. Specifically, patent document 4 proposes a technique in which the warpage of the growth surface of a base substrate is regulated so as to have a radius of curvature of 2 m or more to thereby inhibit cracking.